Appealing against poor valuation

Hi there, I was wondering if someone might be able to help:

We are attempting to get equity release for a property but it was undervalued by quite a fair bit.

There were also inaccuracies on the valuation report.

Are there any legal clauses to prevent loan companies from undervaluing houses like this?

I'm having trouble proving that the house is worth more as not many houses similar to ours have sold in the last year.

Many thanks,